Telemetry monitoring is one important trend for the development of products in the field of medical equipment. Since the electromagnetic wave signal strength fading occurs upon encountering obstacles or generation of inter-wave interference, there exists correspondingly a receiving blind zone for the wireless signal carrying telemetry information. Therefore, one of the key techniques for the enhancement of product competitiveness is to improve the receiver's capability of channel fading resistance so as to eliminate the influence of wireless signal fading on data transfer.
Thus the antenna diversity technique comes into existence. With respect of the form, the diversity technique includes time diversity, frequency diversity, code diversity and spatial diversity. The spatial diversity receiving technique is the technique of providing a plurality of spatial signal receiving channels by using a plurality of appropriately arranged antennae, and always selecting the channel with a stronger signal to perform data transfer, so as to substantially eliminate the influence of the signal fading.
The prior art solutions based on the spatial diversity techniques are mainly classified into two types. In the case of a two-antenna configuration, the first type is to dynamically monitor the receiving signal strength of the current antenna input channel, and switch to the other antenna compulsively where the strength is lower than a preset threshold value. This solution requires to provide only one received signal strength indication (referred to as RSSI hereinafter) unit for two antenna input channels. The second type is to simultaneously dynamically monitor the receiving signal strength of each of the two antenna input channels, and automatically select the antenna with a stronger signal strength as the input channel. This solution requires to provide two RSSI units independent of each other for the two antenna input channels, respectively.
The prior art technique as mentioned above has the following disadvantages. The channel switching according to the first solution is rather passive, as it is activated only when the signal strength is very weak, while communication error code may have already been generated at this point of time. Furthermore, the switching likely occurs in the transfer process of a data frame, and therefore the transient interference caused by the switching may give rise to additional error codes. In addition, since the signal status of the other channels remains unknown before switching, it is likely that the post-switching signal strength is even weaker, which leads to more serious error codes.
The second solution overcomes the disadvantage of the first solution in that it always selects the antenna with stronger signal strength as the signal input channel. However, the total number of the RSSI units is twice that of the first solution, and every two RSSI units have to be connected to a different antenna input and the receive frequency should always be synchronous As such, these two RSSI units ask for self-contained input, mixed-frequency and mid-frequency circuits, which complicates the realization of this solution and incurs a fairly high cost. In addition, the second solution switches the channels compulsively according to the result of signal strength comparison as the first solution, and thereby cannot address the problem of error code due to transient switching interference.